DETROIT — Caleb Thielbar decided to switch up his routine to start incorporating more running into it after not doing much over the past three years.

Why?

“To prevent lower body injuries,” Thielbar said.

The irony is that Thielbar ended up suffering the type of lower-body injury that he was hoping to prevent, straining his left hamstring while running during spring training.

He didn’t pitch in a spring game and missed the first couple of weeks of the Twins’ season. But on Sunday, he was finally activated from the injured list after a pair of rehab outings in St. Paul.

“It was frustrating. More just frustrating how it happened, me being an idiot,” Thielbar said before Sunday’s game. “I learned my lesson, so thankfully I only missed 10 or 12 games, whatever it ended up bein. So plenty of time to end up having a good, productive season and help these guys out.”

Thielbar has been one of the Twins’ most productive relievers over the past four seasons, though his return Sunday did not go quite to plan.

He allowed a home run, a hit that likely should have been ruled an error, and another single before departing Sunday. Three earned runs were charged to him, and he recorded just one out in the Twins’ 4-3 loss to the Detroit Tigers.

“He’s a good pitcher, so yeah, it’s not what you expect,” manager Rocco Baldelli said. “But he’s not going to make any excuses. It’s his first outing back in the big leagues. Been out a while, and sometimes it takes time to be sharp.”

The 37-year-old lefty is now one of three southpaws in the Twins’ bullpen, more than they usually carry. To make room for him on the roster, the Twins optioned Jorge Alcala, rather than rookie lefty Kody Funderburk. Steven Okert, the other lefty in the bullpen, has been seeing some high-leverage opportunities that likely would have been Thielbar’s in his absence.

Thielbar provided a fresh arm for a bullpen that was heavily taxed during Saturday’s doubleheader. Alcala pitched two innings, picking up the win in the first game. He had yet to allow an earned run in 8 1/3 innings at the time he was optioned back to Triple-A.

“I talked to Jorge about a couple of things specifically — working ahead of hitters, coming in with his best stuff — but he did a good job for us,” Baldelli said. “We just know over the course of the season, the bullpen guys will take the brunt of some of this at times when we do need to activate someone or bring a new arm up.”

Wallner off to tough start

Matt Wallner sat most of Sunday — he pinch hit and walked in the ninth inning —  amid a slump that has seen him strike out in 16 of his 32 plate appearances this season.

The Twins’ left  fielder has collected just two hits in 24 at-bats. One, a home run, came on Saturday against Zach McKinstry, a Detroit infielder. But Baldelli has often praised Wallner for his ability to make adjustments. And though he has had a difficult start to the season, he pledged his faith in the outfielder.

“He’s had a tough go of it. I don’t think there’s any way around the fact that he’s either not seeing it well or the swing doesn’t feel right to him,” Baldelli said. “He’s a player that we believe in, though. … This happens to everyone at one point or another in their career and in their path. It’s not an easy thing to go through, but he can handle it. He’ll be OK.”

Briefly

The Twins will send Louie Varland to the mound Monday in Baltimore, where they will see Major League Baseball’s top prospect, Jackson Holliday, who was recently called up by the Orioles. Chris Paddack and Pablo López are also lined up to face Baltimore.